In Functional Programming, what is a functor?

Question

I\'ve come across the term \'Functor\' a few times while reading various articles on functional programming, but the authors typically assume the reader already understands

Answer 1

Put simply, a functor, or function object, is a class object that can be called just like a function.

In C++:

This is how you write a function

void foo()
{
    cout << "Hello, world! I'm a function!";
}

This is how you write a functor

class FunctorClass
{
    public:
    void operator ()
    {
        cout << "Hello, world! I'm a functor!";
    }
};

Now you can do this:

foo(); //result: Hello, World! I'm a function!

FunctorClass bar;
bar(); //result: Hello, World! I'm a functor!

What makes these so great is that you can keep state in the class - imagine if you wanted to ask a function how many times it has been called. There's no way to do this in a neat, encapsulated way. With a function object, it's just like any other class: you'd have some instance variable that you increment in operator () and some method to inspect that variable, and everything's neat as you please.